1. Field of the Invention
The present invention generally relates to medical devices for determining a measure of hemodynamic parameters such as the cardiac output, the stroke volume, or the contractility of a patient and in particular to implantable medical devices such a pacemaker or cardioverter/defibrillators (ICDs) and systems including such a device and an external programmer for determining a measure of a hemodynamic parameter such as the cardiac output, the stroke volume, or the contractility of a patient for use, for example, in trending heart failure or in an AV/VV optimization scheme.
2. Description of the Prior Art
Intracardiac impedance variations has been found to reflect the cardiac function and may hence be utilized for heart therapy in an implantable medical device such as a heart stimulator. In particular, the cardiac impedance has been found to be of great therapeutic value since the cardiac impedance correlates well with hemodynamic parameters such as, for example, cardiac output and stroke volume.
Implantable medical devices of this above-mentioned general type is known. For example, according to United States Patent Application Publication No. 2005/0215914 the ventricular impedance is used as a measure of end-diastolic volume in order to detect heart failure. The measured impedance, which is in inverse proportion to the ventricular end-diastolic value, is compared with threshold values representative of the onset and severity of heart failure and for comparison against previously detected ventricular end-diastolic values of the patient for use in tracking the progression of heart failure over time.
United States Patent Application Publication No. 2004/0078058 describes a heart stimulator including an analyzer that analyzes at least one predetermined parameter of an average impedance morphology curve for use for the control of the stimulation. A parameter having a value that is primarily dependent on the left ventricular ejection is used. The parameter may, for example, be the integrated area below the averaged impedance morphology curve versus time, maximum or minimum value of the average impedance morphology curve, or the difference between the maximum and minimum value of the average impedance morphology curve.
U.S. Pat. No. 5,843,137 discloses a method and apparatus for automatic determination of a pacing stimulations threshold. Values such as maximum, minimum and direction values that characterize the morphology of the impedance waveform is used to discriminate between capture and loss of capture.
U.S. Pat. No. 6,134,472 describes an implantable heart stimulation device that measures electrical impedance to obtain a measure of the ventricular filling. The impedance is measured at the time when the impedance reaches a peak value, which occurs at an approximately fixed time about 250 to 300 ms after the stimulation pulse, and immediately prior to emission of a stimulation pulse. The difference between these two measurement values provides a measure of the stroke volume. This procedure requires a precise synchronization between the impedance measurements and the stimulation pulses in order to provide a measure of the stroke volume and accordingly it may be sensitive to disturbances and/or time delays.
Thus, there is a need for an improved implantable medical device and method for such a device that are capable of providing a reliable and accurate measure of hemodynamic parameters such as stroke volume, cardiac output, or contractility.